Intelligent Intercooler Water Spray - Part 1

Developing the world's best DIY intercooler water spray control system.

To not waste water while still being very effective, an intercooler water spray needs to be use an intelligent, adaptive-learn control system. Here we background the development of the superb (and very cheap!) AutoSpeed / Labtronics intercooler water spray controller. But before we get into the nuts and bolts of the intercooler water spray, you gotta understand how an intercooler works in the first place. Reckon you know that already? You might be surprised....

Intercooler Functioning

It seems straightforward enough. An intercooler acts as an air/air radiator for the intake air, cooling it after the compression of the turbo has caused it to get hot. The compressed air passes through the intercooler, losing its heat to the alloy fins and tubes that form the intercooler core. This heat is immediately dissipated to the outside air that's being forced through it by the forward movement of the car. (We'll get to water/air systems in a moment.)

The trouble with this analysis is that - for a road car - it is not entirely correct. Huh? So what actually happens, then?

I've watched turbo engine intake air temperatures every day for the last 11 years. All have been displayed on digital gauges permanently stuck to the dash of the six different turbo road cars that I have owned - a Commodore VL turbo, Daihatsu Mira turbo, Subaru Liberty (Legacy), C210 RB20DET Skyline, R32 Nissan Skyline GT-R, and an Audi S4. This list includes cars with boost pressures of up to 21 psi (the Mira), air/air intercoolers (GT-R, VL, S4) and water/air systems (Mira, Liberty, C210). And - irrelevantly - the list also includes turbo three, four, five and six cylinders! You might say that I've watched intake air temperature gauges on turbo road cars for more than a quarter of a million kilometres.

So what?

The reason for this build-up is that what follows is likely to be seen as incorrect by many people. For example, someone who measures intake air temps while running a turbo intercooled car for a power pull on a dyno, or who drives it around the block, or who sits back and simply theorises, is almost certain to think that what follows is wrong. But, it isn't.

Heat Sinks

In road cars, intercoolers act far more often as heat sinks rather than as radiators. Instead of thinking of an intercooler as being like the engine coolant radiator at the front of the car, it's far better to think of it as being like a heatsink inside a big sound system power amplifier. If an electric fan cools the amplifier heatsink, you're even closer to the mark.

In a sound system amp, the output power spikes are always much higher than the average power - for example, big output spikes are caused by the beat of a bass drum. Each time there's an output power spike, extra heat is generated by the output transistors and dumped into the heatsink. But because the heatsink has a large thermal mass (it can absorb lots of heat with only a slight temperature rise) the actual working temperature of the transistors doesn't increase much. And because the fan's hard at work blowing air over the heatsink, this inputted heat is then gradually transferred to the atmosphere, stopping the heatsink temp from continuously rising.

Importantly, because the power spike is just that (a spike, not a continuous high output signal), the heat that's just been dumped into the heatsink is dissipated to the air over a relatively long period. This means that the heatsink does not have to get rid of the heat at the same rate at which it is being absorbed.

Now, take the case of a turbo road car. Most of the time in a turbo road car there's no boost occurring. In fact, even when you're driving hard - say through the hills on a big fang - by the time you take into account braking times, gear-change times, trailing throttle and so on, the 'on-full-boost' time is still likely to be less than fifty percent. In normal highway or urban driving, the 'on-full-boost' time is likely to be something less than 5 per cent!

So the intercooler temperature (note: not the intake air temp, but the temp of the intercooler itself) is fairly close to ambient most of the time. You put your boot into it for a typical quick spurt, and the temperature of the air coming out of the turbo compressor rockets from (say) 40 degrees C to 100 degrees C. However, after it's passed through the intercooler, this air temp has dropped to (say) 55 degrees. Where's all the heat gone? Traditionalists would say that it's been transferred to the atmosphere through the intercooler (and some of it will have done just that) but for the most part, it's been put into the heatsink that's the intercooler. The temperature of the alloy fins and tubes and end tanks will have risen a bit, because the heat's been stored in it. Just like in the amplifier heat sink. Then, over the next minute or so of no boost, that heat will be transferred from the intercooler heatsink to both the outside air - and also to the intake air going into the engine.

Real Life Stuff

All getting a bit complicated? OK, let's take a real-life example. In South Australia (where I live) there's a good, four lane road that climbs a very large hill (for locals - it's Willunga Hill). Many less powerful cars struggle at full throttle to crest the top of the hill at 110 - 120 km/h. Others can manage only 80 or 90 km/h. My Skyline GT-R could top the hill at about 200 km/h, with full throttle and full boost being used for perhaps the prior 30 seconds.

(Only 30 seconds? Another point often forgotten in this debate is: how long can you hold full throttle in a turbo road car? Answer: in the real world, not very long!)

Using a quick-response K-Type thermocouple working with a high-speed digital LED dash meter, I could watch intake air temp, measured to one decimal place. From the bottom to top of the hill, the intake air temp never rose by more than 2 degrees C, and in some cases, often actually fell a very small amount! However, after the top of the hill had been reached and the throttle was lifted, the intake air temp would then typically rise by 5 or even 10 degrees. Why? The stored heat was being dumped back into the engine's inlet air as well as to the atmosphere.

In my Audi S4 (again equipped with a K-Type thermocouple intake air temp display), the smaller intercooler means that once over the top of the same hill, the intake air temp rises by a greater degree - an increase as high as 20 degrees C in fact.

Another example. In my high-boost Mira Turbo I ran a water/air intercooling system. The water/air heat exchanger comprised a highly modified ex-boat multi-tube copper heat exchanger, with a few litres of water in it. An electric pump circulated the water through a separate front-mounted cooling core. Intake air temp was measured using a thermistor and a dedicated LCD fast-response meter.

In normal point-and-squirt urban driving, the intake air temp remained the same with the intercooler pump switched either on or off! Why? Because when the car was on boost, the heat was being dumped into the copper-tube-and-water heatsink, and when the car was off-boost, this heat was fed back into the (now cooler) intake air flow. Of course, if I was climbing a long hill (ie on boost for perhaps more than 15 seconds) the pump needed to be operating to give the lowest intake air temps. But even in that tiny car, 15 seconds of constant full boost would achieve over 160 km/h from a standstill...

The latter shows why water/air intercooling in road cars is so successful - but why most race cars use air/air intercooling. Water has a very high thermal mass, so easily absorbing the temp spikes caused by a road car's on/off boost driving. However, race-style boost (say on full boost for 70 per cent of the time) means that the system has to start working far more as a real-time heat transfer mechanism - which is best done by very large air/air intercoolers.

The key point is that typical road car air/air and water/air intercooling systems act as heat sinks during boost periods at least as much as they act as heat transfer mechanisms.

Water Sprays

In a way the point being made in this article is obvious. When you're testing a car's intercooler, it's common to occasionally stop the test and feel the temperature of the core. If it's hot you know it isn't working very well. And that's because you automatically realise that it is primarily acting as a heatsink! If it was just a radiator, the hotter it was, the better it would exchange heat with the ambient air.....

So, that's a pretty big prelude to the topic of intercooler water sprays, isn't it? But if you've been following along, you'll see that having a spray that switches on only when the engine's on boost is not very helpful. Why? Because you really want the intercooler core to be cooled before the engine comes on boost, giving a lower temperature heatsink into which more heat can be dumped. This is exactly the same philosophy that sees some turbo drag cars having their intercooler cores sprayed with nitrous oxide to cool them down before a run. If the water spray operates only when on boost, the spray operates too late. But there are also other factors to consider.

Let's look at a real-life example of intercooler water spray control. In the Skyline GT-R, I built and installed a sophisticated dual-nozzle water spray for the large standard air/air intercooler. I initially triggered it from the K-Type thermocouple dash digital display (the one discussed earlier), so that whenever the intake air temperature exceeded 40 degrees, the spray would operate. Result? One rapidly empty 12 litre water tank; no discernible change in inlet air temp! The reason that the water tank emptied so fast is that in urban driving, the intake air temp is often high. This is because the small amount of air being drawn into the engine is rapidly heated by the high under-bonnet temperatures. This meant that in the Skyline's case, the spray was operating even with the car idling in traffic - and the intake air temp did not drop as a result of the spray working. Using a control system consisting solely of an intake air temperature switch simply doesn't work in a road car.

Next, I used a boost pressure switch wired in series with the temperature switch, so that there had to be positive manifold pressure (ie boost) and the intake air temp also had to be over 40 degrees before the spray would operate. Result? Much reduced water consumption, no measurable change in intake air temp! So, using both intake air temperature and boost inputs didn't work very well. Why? Because by the time the spray started to evaporate and cool the heatsink, the boost event was usually all over!

Let me stress again: on a racetrack, or on the dyno, I'm sure that both approaches would have reduced the intake air temp. The water spray would have been operating very frequently or even continuously - therefore, after the first few throttle applications, the spray would in fact be operating before the next dose of throttle. But we're talking about the real world here, not artificial tests. (And another point to remember about the chassis dyno testing of intercoolers - when compared with the car on the road, the ambient airflow passing through the core is wrong in its characteristics of turbulence, speed, temperature and pressure. Great test - I don't think!)

Another real-world problem of intercooler water sprays is that large water tanks are heavy, and also a pain to keep re-filling. So - while water's certainly cheap - a good intercooler water spray system also needs to be as conservative as practically possible in its water use. A single smallish boost squirt in traffic - the intercooler temp low and with plenty of following time to dissipate the heat - does not require that the water spray operate. To spray water in this situation is to simply waste it.

So the key intercooler water spray activation questions become:

How do you set up a control system so that the spray operates before you get on the loud pedal?

And how do you configure it to use as little water as possible?

Intelligent Controls

Obviously there is no way that an intercooler water spray controller can precisely work out what the driver is going to do, before he or she actually does it. For example, the controller can't look ahead during country road driving, realise that a high-speed overtaking manoeuvre is coming up, and cool the intercooler in preparation for it. Nor can it pick when a traffic light grands prix is about to happen - and then react accordingly!

But what it can do is to monitor the behaviour of the driver, picking how hard he or she is driving. If the driver is using lots of power, the water spray can trigger early and maintain a spray for a long period - even during gear changes and on trailing throttles. But if the driver uses just a single burst of throttle, the water spray controller can ignore it, knowing that the intercooler heatsink won't even have started rising in temperature. And if the temperature of the intercooler core is constantly being compared with ambient temperature, an even better picture of what's going on can be realised.

The Labtronics / AutoSpeed intercooler water spray controller actually analyses driver behaviour and intercooler temperatures and then decides exactly when and for how long to work the spray. This approach overcomes most of the control problems listed, and at a very competitive price.